Carb Synching 101

[DGXSER]

There always seems to be someone coming on to the site who, much like me when I first arrived, is fairly new to bike mechanics and or some of the more technical aspects of wrenching on bikes and multiple carb set ups.

For those of you who are old hats at synchronizing the four carb setup, this will seem quite basic. So just look at the pictures and find things to critique about my carbs. I wanted to make it useful for the less familiar with mechanics as well as these carbs, so bear with me in the parts that seem too basic.

I decided it might be worth the time to go over exactly what carb synching is and how it physically functions on the carbs. This may help those learning the process and the technique to understand what it is they are doing and why the sequence is important.

Here is a photo of the engine side of the carbs where the synch screws are located. You may note this is the early or 78-79 model carbs. The synch screws are in the same location on the 80-81 style, and the process is identical.


The Red arrow is the no 4 synch screw, the green arrow is the no 2 to 3 synch screw, and the yellow arrow is the no 1 synch screw. The naming will become clearer as we go through the process.

Start by setting the idle to about 1100 RPM. This is done by turning the idle adjustment screw. Since this is a 78-79 set, the idle screw is on the air intake side, shown with a green arrow in the photo below. The other green arrow shows the metal plate that gets raised or lowered as the idle adjustment screw is turned.


The 80-81 carbs have the same slotted kind of screw on the bottom of the engine side, and it adjust a similar plate.

In this photo the green arrows show the same plate as it comes up and mates to the no 3 carb butterfly shaft. So adjusting the idle screw DIRECTLY opens and or closes the no 3 butterfly valve.


All of the green arrows are pointing to the same bracket or single piece of steel. This is the same plate that the throttle cable attaches to, so it also adjust the number three butterfly valve DIRECTLY. I emphasis directly, because it is important to understand that nothing else effects the no 3 butterfly valve opening or closing but the throttle cable and or the idle mixture screw. The synch screws have ZERO effect on how much the no 3 butterfly valve is open or closed.

Now that you have the idle adjusted, you can start by synching either the no1 or no 4 carbs, it is really not important which side you do first, but for continuity purposes, I am going to no 4 first.

In the photo below, the green arrows show both ends of the no 3 butterfly shaft. So adjusting the idle screw moves both ends at the same time. The ends of the shafts are square, and the plates shown have square holes that go over them. The blue arrows show plates on both ends of the no 2 butterfly shaft, the red arrow is the bracket on the end of no 4 butterfly shaft, and the black arrow with a yellow border, is the bracket on no 1 butterfly shaft.


Now is where the adjustment begins, turning the synch screw between no3 and no 4 carbs with a red arrow pointing to it in the photo below, adjust how open or closed the no 4 butterfly valve is in relation to the no 3 butterfly valve. Adjusting the screw lets the springs push up or down the u shaped bracket with a red arrow pointing to it, setting the no 4 butterfly more open or closed. Again, turning that screw does not change how open or closed the no 3 butterfly valve is AT ALL.


Now I know when you adjust it the vacuum being drawn by no 3 does change. That is simply the marvel of physics adjusting how the cylinders work together since they attached to the same crank and cam shafts.

Once you have no 3 and 4 vacuum the same (or dang close to it anyway), you can move on to the synch screw between no 1 and no 2 carbs. The yellow arrow in the photo below shows that same sort of U shaped bracket that is attached to the no 1 carb butterfly shaft. SO turning the synch screw between no 1 and no 2 carbs only changes how open or closed the no 1 butterfly valve is in relation to the no 2 butterfly valve. The blue arrow is pointing to the bracket on the no 2 carb butterfly shaft. It also does not move when the synch screw is turned. The adjusting of this synch screw also has no effect on how open or closed the butterfly valves are on no. 3 or no. 4 carbs.


So now you have no 3 and 4 at the same vacuum, and you have no 1 and no 2 at the same vacuum. You guessed it, time for the synch screw between no 2 and no 3. Sorry for the lack of color coordination in this photo, I used green for plate on the other side of the carb 2 butterfly shaft. In the photo below you can see that adjusting this screw only changes how open or closed no 2 is in relation to no 3 butterfly valve position. However, no 1 is coming along for the ride when you adjust this screw. As you adjust the position of the no 2 butterfly valve, it changes the position of the brackets on both ends of the shaft. So moving the bracket on the no 1 carb side, also moves the no 1 carb butterfly valve, they move as a pair based upon that synch screw setting we did on the step above.


So now you should have all four carbs pulling the same vacuum (Or dang close to it anyway). The idle may have changed, so you will need to adjust the idle back to around 1100 RPM.

So what happens when you adjust the idle screw now that your all in sych??? Well, adjusting the idle screw DIRECTLY changes the position of the no 3 butterfly valve. However, it also adjust the position of no 4 and no 2 via those synch screw assemblies. And moving no 2 moves no 1 via that synch screw assembly. You directly adjust no 3 butterfly valve position with the throttle cable or the idle screw, and the others come along for the ride.

You may need to go back over the synch if your idle was high or low substantially when you finished synching.

You will also want to adjust mixture at this time, either by ear, by color tune, or by watching the vacuum in each cylinder and adjusting the mixture to achieve the highest vacuum reading in each cylinder. And once you finish adjusting the mixture, you will want to repeat the synching process.

Hope this helps some of you understand what the synching process really is and how it works in a physical sense anyway.

[Tat2demon]

Excellent write up. Im sure this will help a lot, and it has shiny pictures for people like me!

[MarkD]

DG-- Thanks for the very understandable and helpful instructions on the sync job. It will make life easier for a lot of members.

[BA80]

The one thing you didn't show or mention Don is that if the carbs have been removed and/or overhauled is the bench sync. If they aren't pretty close to begin with it can be a bear to get the sync done.

Recent experience speaking here.

just sayin'

[Ken Talbot]

Nicely done!

[H4rdcore]

Even i can understand that..and im going to have to do it...

However heres my question..***NEWBIE ALERT***

Why do you have to sync the carbs when you remove them and clean them?
If you dont touch the idle screw or butterflys should they not be the same as when they were on the bike?
Just curious..if you dont mess with the setup and your just removing the jets and maybe your floats and cleaning it out what changes?

[crazy steve]

One minor clarification, and an alternative for carb order....

One, the idle speed screw is the only 'sync' adjustment for #3, but it should be noted that changing that opens/closes all four carbs.

Two, you can also sync the carbs like this: adjust 4 to 3, then 2 to 3/4, then 1 to 2/3/4. Either way works, but I've always had to slightly readjust one of the previously adjusted carbs as they get 'closer' together (i.e. 3/4 are good, but bringing in 2 causes 3/4 to go slightly out) with either method, but I seem to get smaller 'changes' by doing them singly instead of pairs.

If there's agreement on this, next stop in the tech section....

[crazy steve]

Why do you have to sync the carbs when you remove them and clean them?
If you dont touch the idle screw or butterflys should they not be the same as when they were on the bike?
Just curious..if you dont mess with the setup and your just removing the jets and maybe your floats and cleaning it out what changes?

They can get out of sync just from use; the factory manual recommends checking them every 3200 miles, although that's probably overkill. So if they're bad enough that they needed cleaning, they've probably been on long enough to need a sync...

[Schming]

To add to Steves statement, as in "Why to synch ?", achieving balance of the vacuum between cylinders is also dependent on the condition of the rest of the moving parts in the engine that will affect flow.

[motoman]

Even i can understand that..and im going to have to do it...

However heres my question..***NEWBIE ALERT***

Why do you have to sync the carbs when you remove them and clean them?
If you dont touch the idle screw or butterflys should they not be the same as when they were on the bike?
Just curious..if you dont mess with the setup and your just removing the jets and maybe your floats and cleaning it out what changes?

You will see while syncing carbs that even the slightest pressure on any of the sync screws changes vacuum. Having said that, it's always a good idea once gone thru sync process, and bike is off to cool(even with fan in front of motor), rolling on and letting throttle snap back several times 'seats' the screw threads. Re-start and notice the change of sync. Re-adjusting again and repeating this action a couple times will 'stabilize' the screw to thread position till all four vacuum readings stay the same after rotating throttle and releasing back to their stops.

[natemoen]

Good write up. One small editorial note though, instead of referring to the carbs as no 1 and no 2 etc. actually write out "number". It could to some of the younger crowd look like you are typing "no one" when you are talking about the number one carb.

Just helps clean it up a little in my mind.

[mikubuilder]

Nice write up....Nate's point about snapping throttle is very important. ...

[DGXSER]

Even i can understand that..and im going to have to do it...

However heres my question..***NEWBIE ALERT***

Why do you have to sync the carbs when you remove them and clean them?
If you dont touch the idle screw or butterflys should they not be the same as when they were on the bike?
Just curious..if you dont mess with the setup and your just removing the jets and maybe your floats and cleaning it out what changes?

What is important to keep in mind is that your adjusting each cylinder to produce the same vacuum, not the butterfly to be open the same amount. Everything that contributes to the combustion process contributes to how much vacuum that cylinder produces. If you change the temp rating of the plug, that will change how much of a bang is produced when it fires, change the timing of the spark, you changed the explosion. Cleaning the carbs effects the air fuel blend your delivering to each cylinder (after all, that's why your cleaning them). That effects the combustion. Any change in the explosion created will also change how hard it pulls a vacuum. Any change in vacuum changes the synchronization.

Hopefully that helps explain why even cleaning the carbs changes the synch. Now, I am not sold the thought that simply removing the carbs and reinstalling them changes anything, however, it is as good an excuse as any to check the synch. And yes, just running the engine causes wear and carbon build up here and there, plugs do weaken with age, etc, such that it is never a bad idea to check the synch.

[DGXSER]

I suppose I should have been clearer that my intention was not to fully document the procedure of synchronizing the carbs, (In fact, there are several videos made by members that show that process.) but to explain the physical relationship of the parts and how they physically effect each other. Which is why I did not go into the bench synch concept Greg, I considered that well covered and explained in the Carb 101 thread. As I did elude to, although adjusting one screw does not physically change the butterfly valve position of all the carbs, changing the vacuum of one does actually effect the vacuum of all of them at least a little.

One, the idle speed screw is the only 'sync' adjustment for #3, but it should be noted that changing that opens/closes all four carbs.

But then technically, if it is not changing the relative position of one butterfly valve to another, or the vacuum of one cylinder relative to another, it is not actually synchronizing, just adjusting the idle speed. You do bring up a good topic though, the point of synchronizing is not to achieve any set vacuum reading, meaning your goal is not to achieve 22 across the board, or 30, or any other set number. It is simply to achieve the same vacuum on all four. Changing the idle speed will change the vacuum on all four, but if it is a minor adjustment, it should effect all four relatively equally if they have been synched. However your not adjusting the idle speed to achieve a given vacuum reading, only to set the RPM of the engine.

Two, you can also sync the carbs like this: adjust 4 to 3, then 2 to 3/4, then 1 to 2/3/4. Either way works, but I've always had to slightly readjust one of the previously adjusted carbs as they get 'closer' together (i.e. 3/4 are good, but bringing in 2 causes 3/4 to go slightly out) with either method, but I seem to get smaller 'changes' by doing them singly instead of pairs.

Yes, of course, you can do them in any order you want, just as long as they are all in synch when you consider yourself finished. Again, technically, any time you adjust the synch screw between the 2 and 3 carb you ARE adjusting them in pairs. When you change two, you change 1, it comes along for the ride if you want it to or not.

And to your point Nate, on the no 1, no 2 ....issue, you have a good point. I found I was going back through trying to put a period after the no for number, even considered going back through it to change those no. to #. It is not just the young who might get confused, I wrote it and it might confuse me.

As to the other items referring to the process of synching, all good points. I do pop the throttle a couple times after I think I am finished with each synch screw.

[thebottle]

Thanks, great write up.